This invention relates to electrodeposited copper foils such as are used to make "printed" circuits. In particular, the invention relates to a copper foil having a high tensile strength even after heating and characterized by having a low profile matte side and a unique crystal orientation, which provides more precisely etched circuit lines.
Electrodeposited copper foils are made by electrolysis of a copper solution. Typically, copper metal is deposited on a rotating metal drum which serves as the cathode. After the desired thickness has been achieved, the copper foil is removed from the drum and given post-treatments to protect it and to improve adhesion to a substrate. Then, the foil is wound into rolls for shipment to the user, where it is laminated to an insulating substrate, such as a glass fiber reinforced epoxy resin, and then photoimaged and etched to produce the desired circuit pattern. The etching process is critical to the actual performance of the circuit. Ideally, only copper not intended to be a part of the circuit pattern should be removed by the chemicals used to etch the copper. In fact, the copper is removed irregularly and instead of circuit lines having sharply defined sides, they are typically trapezoidal in shape. The top of the circuit lines are usually narrower than intended and the bottom wider. Furthermore, small amounts of copper may be left embedded in the substrate which are in electrical contact with the circuit lines. While some of these deficiencies can be countered by the circuit designer, who expects that etching will not be entirely accurate, nevertheless, one can appreciate that inaccurate etching will limit the spacing of the circuit lines. When they are too close together, short circuits can occur or the current passing through one circuit line can influence adjacent circuit lines. Consequently, accurate etching is important if increased circuit densities are to be achieved. However, the results of etching are largely influenced by the structure of the copper foils.
The invention is particularly useful in the tape automated bonding process (TAB), but it has application in conventional printed circuit boards. The need for higher circuit densities has been growing in TAB applications and next-generation TAB tapes should accommodate 320 channel, 50 .mu.m pitch (spacing) circuits. Thus, more precise etching becomes ever more essential, to provide adequate insulation between the circuit lines and to produce sharply defined profiles.
Another problem to which TAB tapes are subject is that in multi-layer tapes the high temperatures used (about 180.degree. C.) causes a drop in the tensile strength of the copper. This can result in bending of the inner leads. Thus, higher tensile strength copper foils are desirable in addition to the improved etching character discussed above.
Three related patents can be mentioned as having some relevance to the present invention. They are U.S. Pat. Nos. 5,403,465; 5,421,985; and 5,431,803. In each patent, conditions are changed in the electrolytic process in order to change the tensile strength, the elongation, and the roughness of the copper surface. In the '803 patent it is disclosed that under certain conditions uniform randomly oriented grains can be made and that columnar crystals can be avoided. While such foils proved improved tensile strength they have not been found to provide improved etchability for making fine lines and spaces.
The present inventors had as an objective improving the etching factor of copper foil, to reduce the surface roughness on the matte side of the foil, and to increase tensile strength. They have accomplished their objectives, as will be seen in the discussion below.